Separation vessel with enhanced particulate removal

ABSTRACT

A separation tank for crude oil. Fluid enters an inlet section of a center column of the tank via an offset inlet pipe so the fluid enters swirling. Solids that settle in the inlet section are removed by a center column drain and a solids removal system. Free gas rises and exits from the top of the tank. Liquid flows out of the center column via a diffuser that spirals the fluid evenly toward the wall of the tank where oil coalesces and wicks upward. Liquid flows downward around two flow diverting baffles where more oil coalesces and wicks upward via an oil conduit into the oil layer. The water flows under the lower flow diverting baffle and exits the tank through the outlet section. A large circular oil collector weir uniformly removes oil from the oil layer. Interface draw offs located below the oil-water interface remove excess BS&amp;W.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is a separation vessel for separating gas,sediment, and water from crude oil for oil production that containssignificant amounts of water.

2. Description of the Related Art

With oil prices hovering around $85-$100/barrel, current economicsstrongly favor separating and selling every drop of crude oil possible.Water production now dominates many oilfield operations, and too muchoil remains entrained in it. The conventional API gun barrel separatortanks are the type of separation vessels that are often used to try toseparate that oil. Those tanks were designed to remove small quantitiesof water from large quantities of oil, not small amounts of oil fromlarge quantities of water. Today's high water cuts suggest that theseold industry workhorses may be obsolete when large volumes of water areinvolved.

The present invention addresses this problem with a more sophisticated,a more complex, and a more expensive type of separator. However, attoday's oil prices, the initial cost of installation of this moreexpensive type of separator is recovered in just a matter of days by thedirect benefit of increased oil recovery achieved by this new separatordesign over the conventional gunbarrel vessels currently in use.

Also, there are other indirect cost savings associated with disposal ofwater effluent from the present invention verses disposal of watereffluent from the conventional gun barrel vessel tanks currently in use.The oil that exits with the water effluent from the inefficientconventional gunbarrel vessels is disposed of with the water effluentinto injection wells or disposal wells. The oil contained in that watereffluent has a tendency to plate out on the tubular, the well liner, thewell bore and the formation rock of the disposal well. Because the oilis water-insoluble, as it coats the formation face, it begins torestrict or plug the flow of water flowing from the well to theformation. Most of the suspended solids in the water accumulate in thisoily material, increasing the volume of the deposit and causing evenmore plugging. This oily residue tends to build up in the formationwithin a few feet of the well bore and on the formation face, formingimpervious flow paths that eventually cause injection pressures to climband injection rates to decline.

As injection rates decline, it is common practice to stimulate thedisposal well, often using a dilute solution of hydrochloric acid orother common stimulation solvents, usually with added surface activechemical ingredients. After the first stimulation, the result is thatthe well is returned to near its original injection rate and pressure.However, it is also common that after the first stimulation, injectionrates fall off and injection pressures increase more rapidly thanbefore. This situation becomes more severe after each subsequentstimulation effort until a point of diminishing returns is reached.Eventually, when stimulation efforts fail and the disposal well bore isobviously damaged beyond reclamation, it is then necessary to re-drill,sidetrack and recomplete the existing disposal well, or to drill a newdisposal well. The costs for these more drastic measures range from$500,000 to $3,000,000. This is the indirect cost of poor water qualityin the effluent from the oil water separators that are in use today.

With such staggering direct and indirect costs, it seems prudent to takepositive steps to capture and sell as much of the entrained oil aspossible in the crude oil stream, and to take steps to prevent wellplugging from any and all other sources of contaminants such as solids,bacteria, etc.

One step is to select oil-water separation equipment that actuallyseparates all physically separable oil from the produced water. The goalof the present invention is to provide a 20-30 fold increase inseparation efficiency over conventional gunbarrel separating tanks.Conventional gunbarrel tanks will be only 3-5% hydraulically efficientat separating entrained oil, whereas the present invention is 60-72%hydraulically efficient at separating the entrained oil. The presentinvention reduces the oil concentration to below 50 ppm in the effluentwater as compared to approximately 300-1500 ppm of oil in the effluentwater emanating from conventional gunbarrel separation tanks.

SUMMARY OF THE INVENTION

The present invention is a separation vessel or tank with enhancedparticulate removal for separating gas, water and particulates fromcrude oil. When the incoming fluid contains gas as one of thecomponents, the tank is provided with an optional degassing bootdesigned to allowing all free gas to separate from the remaining liquid.This avoids the mixing that would occur in the tank if the gas wasallowed to enter with the liquids. The degassing boot may be provided atthe top of the center column or on the top of the tank before the fluidenters the vessel.

Then the incoming production fluid enters the vessel through an inletpipe into a large diameter vertical pipe provided in the center of thetank that is referred to as the center column. The inlet pipe isattached to the center column in an offset manner so that the fluidenters the center column in a circular path to increase retention timewithin the center column.

The center column is divided into two vertical sections: the inletsection and the outlet section. The two sections are separated by ablanking plate. The inlet section extends from the top of the tank tothe blanking plate that is installed within the center column just abovethe lower flow diverting baffle. The outlet section extends from theblanking plate to the bottom of the tank. The blanking plate isinstalled to divide the center column so the inlet fluid cannot flowdirectly to the outlet located below.

Heavier particulates entering with the fluid into the inlet section ofthe center column fall downward within the center column to the blankingplate and are periodically removed either by blow down through a centercolumn drawing or via a solids removal system, such as a Tore solidsremoval system, that is installed within the center column above theblanking plate or via both means.

Any free gas that disengages from the remaining fluid flows upwardwithin the center column and exits the center column via gas holesprovided in the top of the center column and enters into a gas layerlocated at the top of the tank. Excess gas is removed from the tank viaa gas outlet provided in the tank in communication with the gas layer.Also, there may be provided a degassing boot at the top of the centercolumn, or on the top of the tank before the fluid enters the vessel.

The fluid flows out of the center column via a spiral swirl vanediffuser installed in the center column. The spiral swirl vane diffuseris provided with vertical curved or swirl vane baffles. The verticalcurved or swirl van baffles will hereafter be referred to as inletdiverters. Each inlet diverter is secured between a horizontal quietinglower donut baffle and a horizontal quieting upper donut baffle, withadjacent inlet diverters spaced apart from each other. Inlet fluid slotsare provided in the spiral swirl vane diffuser between adjacent inletdiverters. The inlet fluid slots communicate with the inlet section ofthe center column to allow fluid to flow out of the center columnbetween the inlet diverters and into the interior of the tank. The inletdiverters serve to swirl the fluid as it flows out between them. As thefluid exits the center column, it turns from a vertical upward directionwithin the center column to a horizontal outward direction as it exitsthe center column through the spiral swirl vane diffuser to enter aprimary separation zone within the tank.

The spiral swirl van diffuser distributes the fluid within the tank justbelow the oil-water interface through the diffuser's inlet diverters.These inlet diverters are curved to impart a centrifugal force on theliquids, spinning them outward from the center of the tank in an everincreasing radius spiral. This slows the velocity of the inlet fluid andincreases its effective separation time in the primary separation zonejust below the oil-water interface. As the inlet fluid stream slows,smaller and smaller droplets of oil separate and rise the short distanceto the oil layer.

Some oil droplets accumulate on the top of the large area upper flowdiverting baffle which serves also as a huge surface area coalescer. Theupper flow diverting baffle is convex on its upper or top side and isconcave on its lower or bottom side. As the fluid stream spirals outwardaway from the center of the tank, it encounters the interior tank wallthat serves as another large area coalescer. Any droplets of oilattaching themselves to these coalescing surfaces are now no longer inthe water. Instead, they are permanently separated from the water. Asthese surfaces become totally coated with oil, the oil wicks upward,eventually entering the oil layer above, adding to the volume of oilcollected.

The oil layer is designed to provide adequate time for all accumulatingoil to completely dehydrate to typical pipeline specification or better.Uniform oil collection is critical to this function. A very large oilcollector in the center of the tank at the top of the oil liquid layerassures all oil rises uniformly through the entire oil layer, and iscollected around 360 degrees of that layer. The large collector isdesigned with a very large spillover weir. Its height insures a minimumlevel deviation even during periods of very high slug rates. The leveldifferential between the oil outlet and the downstream tank assures thatlarge flow rates of oil can flow out of the tank's oil collector and oiloutlet piping during slug flow conditions. Because of this, it is nearlyimpossible to overflow oil from the tank.

Once the bulk oil has separated from the main flow of inlet water, thewater must turn 90 degrees downward to flow down between the upper flowdiverting baffle and the tank wall. This causes a small measure ofacceleration. As the downward flowing water reaches the outer edge ofthe upper flow diverting baffle, it enters a quadrant of the tank whichis open to full diameter flow. The acceleration velocity creates a mildeddy current that pulls a portion of the water in and under the upperflow diverting baffle. At this point, all fluid flow changes tovertically downward through the entire cross sectional area of the tank.Velocity is now at its slowest, allowing the smallest of oil droplets tocounter flow upward. These droplets rise, coating the concave bottomside of the upper flow diverting baffle. Once coated, the oil canmigrate directly into the oil layer located above through a pipe or oilconduit that extends from the bottom side of the upper flow divertingbaffle up into the oil layer just below the oil collector, thuspreventing re-entrainment of oil in the water. This adds even more tothe volume of oil collected and to the separation efficiency of thetank.

As the clarified water travels downward and nears the bottom of thetank, it encounters a large area lower flow diverting baffle. Like theupper flow diverting baffle, the lower flow diverting baffle is convexon its upper or top side and is concave on its lower or bottom side. Asthe downward flowing water impinges on this lower flow diverting baffle,oil droplets accumulate on its top surface, further enhancingseparation. Additionally, this lower flow diverting baffle forces theflow stream to change directions from vertically downward to nearlyhorizontal again as the fluid turns to flow around the lower flowdiverting baffle.

Now the water is flowing straight toward the inside surface or wall ofthe tank again. As it contacts the tank wall, some of the smallest oildroplets impinge on the wall, coating the wall and are wicked up intothe oil layer above. Once again, separation efficiency is enhanced.

In order to exit the tank, the water must turn downward again to flowbetween the outer edge of the lower baffle and the tank wall. Since thisarea is a fraction of the tank cross section, the water must againincrease in velocity as it turns downward. Any solids in the water atthis point are now aimed directly at, and are being propelled directlytoward, the bottom of the tank.

As the water reaches the outer edge of the lower flow diverting baffle,it must now turn upward more than 90 degrees and flow upward under theconcave bottom side of the lower baffle. Solids, being heavier thanwater, are unable to change directions and thus settle to the bottom ofthe tank. The water flows along the bottom side of the lower flowdiverting baffle where the tiniest droplets of oil have one last chanceto coalesce and attach to this very large surface. Oil accumulating onthe bottom side of the lower flow diverting baffle is allowed to exitthrough oil-dedicated weep holes provided extending through the top ofthe lower flow diverting baffle. That oil exits to the area under theupper flow diverting baffle, migrates upward until it contacts thebottom side of the upper flow diverting baffle and then flows throughthe oil conduit directly into the oil layer.

The water flowing under the lower flow diverting baffle now reaches thecenter of the tank and enters the outlet section of the center columnvia outlet holes provided in the center column below the blanking plateand below the lower flow diverting baffle. Once the water enters thecenter column through the outlet holes, it turns downward and flows downwithin the center column to enter a horizontal water outlet pipe whichdirects the water out of the tank and into an adjustable height waterleg.

The separation tank is fitted with two internal tank drains. The firstinternal tank drain is the center column drain and the second internaltank drain is the set of interface draw offs.

The first internal tank drain is the center column drain. Incoming fluidoften contains some solids. These solids will accumulate preferentiallyabove the blanking plate. A center column drain is provided so theoperator can drain this area. It should be drained frequently until thewater leaving the drain line runs clear.

In order to drain the solids that accumulate above the blanking plate,it may be desirable, in addition to the center column drain, to includea solids removal system such as a Tore® solids removal system to aid thecenter column drain in removing solids from the inlet section of thecenter column. A Tore® solids removal system is a solidshydro-transportation device that utilizes the natural power of a motivefluid, such as water, to mobilize and transport solids, liquids orslurries. Tore® systems are available from PDL Solutions Ltd. located inthe United Kingdom. The Tore® solids removal system includes a waterinlet that feeds water to the Tore® solids removal system and a waterand solids outlet from the solids removal system that drains a mixtureof water and solids out of the inlet section of the center column.

The second internal tank drain is the set of interface draw offs. As oilaccumulates, it is common that some BS&W (basic sediment and water, aka“emulsion”) will accumulate immediately below or at the oil—waterinterface. The BS&W is heavier than pure oil because of the water andsolids contained in it. Therefore, the emulsion will build downward fromthe normal oil-water interface level. About a foot below the normaloil-water interface level are several interface draw offs. These areapproximately 24 inch round horizontal draw off baffles stackedapproximately 4 inches apart so that each interface draw off has anupper draw off baffle separated from a lower draw off baffle with thearea between the two draw off baffles open to the interior of the tank.A draw off pipe is connected to each of the lower draw off baffles andthe individual draw off pipes are connected together and piped to aconvenient elevation near the bottom of the tank where the draw off pipeexits the tank as the BS&W outlet. A BS&W valve is installed to open andclosed the BS&W outlet on the piping. When the BS&W valve on the BS&Woutlet is opened, the BS&W layer flows horizontally between the upperand lower draw off baffles of each interface draw off and out of thetank through the BS&W piping. When either clean water or clean oil isobserved in the sample of the outlet fluid, the BS&W has been removedand the BS&W valve can then be closed.

Although not shown, a water leg is provided to maintain the proper fluidlevel within the tank. The water leg is added at the site ofinstallation.

A water leg is a pipe within a pipe. The clarified water enters throughthe outer pipe and turns upward where it flows in the annular spacebetween the two pipes. The inner pipe is sized for its circumference.The circumference of the outer pipe forms a spillover weir for the waterwith the inner pipe. The height of the top of the inner pipe establishesthe weir that sets the oil-water interface level inside the separationtank. The height of this weir is critical. It is always adjustable,either by removing the upper removable center pipe nipple, or via anexternal adjustment assembly that slides a movable upper section of theinner pipe up and down to change its spillover elevation.

Sand removal systems can also be included in the bottom of the tank.These should be drained daily until clean water is observed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the internal components contained within aseparation vessel that is constructed in accordance with a preferredembodiment of the present invention.

FIG. 2 is top view of the separation vessel of FIG. 1, showing thearrangement of the various internal components.

FIG. 3 is a top view showing the inlet pipe attached to the centercolumn in an offset manner so that the fluid entering the center columntravels in a circular path within the center column.

FIG. 4 is top plan view of the spiral swirl vane diffuser removed fromthe vessel of FIG. 1.

FIG. 5 is a top perspective view of the spiral swirl vane diffuser ofFIG. 4.

FIG. 6 is a bottom perspective view of one of the interface draw offsfrom FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and initially to FIG. 1, there is shown aseparation vessel or tank with enhanced particulate removal 10 that isconstructed in accordance with a preferred embodiment of the presentinvention. The tank 10 is designed for separating gas, water andparticulates from crude oil.

When the incoming fluid contains gas as one of the components, the tank10 is provided with an optional degassing boot (not illustrated) toallow all free gas to separate from the remaining liquid. This avoidsthe mixing that would occur in the tank 10 if the gas was allowed toenter with the liquids. Also, there may be provided a degassing boot atthe top of the center column, or on the top of the tank before the fluidenters the vessel.

Then the incoming production fluid enters the tank 10 through an inletpipe 12 into a large diameter vertical pipe provided in the center ofthe tank 10 that is referred to as the center column 14. Referring nowto FIG. 3, the inlet pipe 12 is attached to the center column 14 in anoffset manner so that the fluid enters the center column 14 in acircular path to increase retention time within the center column 14, asshown by Arrows A and B in FIG. 3.

The center column 14 is divided into two vertical sections: the inletsection 16 and the outlet section 18. The two sections 16 and 18 areseparated by a blanking plate 20 that is installed within the centercolumn 14 just above a lower flow diverting baffle 24 that is attachedto the center column 14. The blanking plate 20 prevents fluid locatedwithin the center column 14 from passing directly between the twosections 16 and 18. The inlet section 16 extends from the top 22 of thetank 10 to the blanking plate 20. The outlet section 18 extends from theblanking plate 20 to the bottom 26 of the tank 10. The blanking plate 20is installed to divide the center column 14 so the inlet fluid cannotflow directly to the outlet section 18 located below.

Heavier particulates entering with the fluid into the inlet section 16of the center column 14 fall downward within the center column 14 to theblanking plate 20 and are periodically removed via a center column drain90 provide above the blanking plate 20 or via a solids removal system28, such as a Tore® solids removal system, that is installed within thecenter column 14 above the blanking plate 20 or by both means.

Any free gas that disengages from the remaining fluid flows upwardwithin the center column 14 and exits the center column 14 via gas holes30 provided in the top 32 of the center column 14 and enters into a gaslayer 34 located at the top 22 of the tank 10 above the gas-oilinterface 37. Excess gas is removed from the tank 10 via a gas vent 35provided in the top 22 of the tank which is in communication with thegas layer 34 within the tank 10. Although not illustrated, there may bea degassing boot at the top 32 of the center column 14 or on the top 22of the tank 10.

The fluid flows out of the center column 14 via a spiral swirl vanediffuser 36 installed in the center column 14. The spiral swirl vanediffuser 36 is provided with vertical curved or swirl vane baffles 38.The vertical curved or swirl vane baffles 38 will hereafter be referredto as inlet diverters 38. Each inlet diverter is secured between ahorizontal quieting lower donut baffle 40 and a horizontal quietingupper donut baffle 42, with adjacent inlet diverters 38 spaced apartfrom each other. Inlet fluid slots 44 are provided in the spiral swirlvane diffuser 36 between adjacent inlet diverters 38. The inlet fluidslots 44 communicate with the inlet section 16 of the center column 14to allow fluid to flow out of the center column 14 between the inletdiverters 38 and into the interior of the tank 10.

Referring now to FIGS. 1, 4 and 5, the inlet diverters 38 serve to swirlthe fluid as it flows out between them. As the fluid exits the centercolumn 14, it turns from a vertical upward direction, as shown by ArrowC, within the center column 14 to a spiraling, horizontal outwarddirection, as shown by Arrows D, as it exits the center column 14through the spiral swirl vane diffuser 36 to enter a primary separationzone 46 within the tank 10.

The spiral swirl vane diffuser 36 distributes the fluid within the tank10 just below the oil-water interface 48 through the diffuser's inletdiverters 38. These inlet diverters 38 are curved to impart acentrifugal force on the liquids, spinning them outward from the centerof the tank 10 in an ever increasing radius spiral, as shown by ArrowsD. This slows the velocity of the inlet fluid and increases itseffective separation time in the primary separation zone 46 just belowthe oil-water interface 48. As the inlet fluid stream slows, smaller andsmaller droplets of oil separate and rise the short distance to the oillayer 50.

Some oil droplets accumulate on the top 52 of the large area upper flowdiverting baffle 54 which serves also as a huge surface area coalescer.The upper flow diverting baffle 54 is convex on its upper side or top 52and is concave on its opposite lower side or bottom 56. As the fluidstream spirals outward away from the center of the tank 10, itencounters the interior tank wall 58 that serves as another large areacoalescer. Any droplets of oil attaching themselves to these coalescingsurfaces 52 and 58 are no longer in the water, and are now permanentlyseparated from the water. As these surfaces become totally coated withoil, the oil wicks upward, eventually entering the oil layer 50 above,adding to the volume of oil collected in the oil layer 50.

The oil layer 50 is designed to provide adequate time for allaccumulating oil to completely dehydrate to typical pipelinespecification or better. Uniform oil collection is critical to thisfunction. A very large, concave, circular oil collector 60 provided inthe center 62 of the tank 10 at the top 64 of the liquid oil layer 50assures all oil rises uniformly through the entire oil layer 50, and iscollected around 360 degrees of that layer 50. The upper edge 66 of thelarge oil collector 60 is designed to serve as a very large spilloveroil weir 68 for oil. Oil from the oil layer 50 that passes over the oilweir 68 and into the oil collector 60 exits the oil collector 60 and thetank 10 via an oil outlet 70 that is attached to the oil collector 60.

The oil weir 68 is tall. Its height insures a minimum level deviationeven during periods of very high incoming fluid slug rates. The leveldifferential between the oil outlet 70 and a downstream tank assuresthat large flow rates of oil can flow out of the tank's oil collector 60and oil outlet 70 during slug flow conditions. Because of this, it isnearly impossible to overflow oil from the tank 10.

Once the bulk oil has separated from the main flow of inlet water, thewater must turn 90 degrees downward, as shown by Arrow E, to flow downbetween the upper flow diverting baffle 54 and the tank wall 58. Thiscauses a small measure of acceleration. As the downward flowing waterreaches the outer edge 72 of the upper flow diverting baffle 54, itenters a quadrant of the tank 10 which is open to full diameter flow.The acceleration velocity creates a mild eddy current that pulls aportion of the water in and under the upper flow diverting baffle 54. Atthis point, all fluid flow changes to vertically downward, as shown byArrow F, through the entire cross sectional area of the tank 10.Velocity is now at its slowest, allowing the smallest of oil droplets tocounter flow upward. These droplets rise, coating the concave bottom 56of the upper flow diverting baffle 54. Once the bottom 56 is coated, theoil can migrate directly into the oil layer located above through a pipeor oil conduit 74 that extends from the bottom 56 of the upper flowdiverting baffle 54 up into the oil layer 50 located just below the oilcollector 60, thus preventing re-entrainment of oil in the water layer76. This adds even more to the volume of oil collected and to theseparation efficiency of the tank 10.

As the clarified water travels downward and nears the bottom 26 of thetank 10, it encounters the lower flow diverting baffle 24 which is asecond large area on which oil can condense. Like the upper flowdiverting baffle 54, the lower flow diverting baffle 24 is convex on itsupper side or top 78 and is concave on its lower side or bottom 80. Asthe downward flowing water impinges on the top 78 of this lower flowdiverting baffle 24, oil droplets accumulate on its top 78, furtherenhancing separation. Additionally, as shown by Arrow G, this lower flowdiverting baffle 24 forces the flow stream to change directions fromvertically downward to nearly horizontal again as the fluid turns toflow around the lower flow diverting baffle 24.

Now the water is flowing straight toward the inside surface or wall 58of the tank 10 again. As it contacts the tank wall 58, some of thesmallest oil droplets impinge on the tank wall 58, coating the wall 58and are wicked up into the oil layer 50 above. Once again, separationefficiency is enhanced.

In order to exit the tank 10, as shown by Arrow H, the water must turndownward again to flow between the outer edge 82 of the lower flowdiverting baffle 24 and the tank wall 58. Since this area is a fractionof the tank cross section, the water must again increase in velocity asit turns downward. Any solids in the water at this point are now aimeddirectly at, and are being propelled directly toward, the bottom 26 ofthe tank 10.

As shown by Arrow J the water reaches the outer edge 82 of the lowerflow diverting baffle 24, it must now turn upward more than 90 degreesand flow upward under the concave bottom 80 of the lower flow divertingbaffle 24. As the solids are heavier than water, they are unable tochange directions and thus settle to the bottom of the tank 10. Thewater flows along the bottom 80 of the lower flow diverting baffle 24where the tiniest droplets of oil have one last chance to coalesce andattach to the very large surface of the bottom 80. Oil accumulating onthe bottom 80 of the lower flow diverting baffle 24 is allowed to exitthrough oil-dedicated weep holes 84 provided extending through the top78 of the lower flow diverting baffle 24. That oil exits to the areaunder the upper flow diverting baffle 54, migrates upward until itcontacts the bottom 56 of the upper flow diverting baffle 54 and thenflows through the oil conduit 74 directly into the oil layer 50.

The water flowing under the lower flow diverting baffle 24 now reachesthe center 62 of the tank 10 and enters the outlet section 18 of thecenter column 14 via outlet holes 86. The outlet holes are provided inthe center column 14 just below the blanking plate 20 and below thelower flow diverting baffle 24. As shown by Arrow K, once the waterenters the center column 14 through the outlet holes 86, it turnsdownward and flows down within the center column 14. As shown by ArrowL, from the center column 14, the water then turns horizontally to entera horizontal water outlet pipe 88 which directs the water out of thetank 10 and into an adjustable height water leg that serves to regulatethe height of the oil-water interface 48 located within the tank 10.

Referring to FIGS. 1 and 2, the separation tank 10 is fitted with twointernal tank drains. The first internal tank drain is the center columndrain 90 that is located near the solids removal system 28. The secondinternal tank drain is the set of interface draw offs 92.

The center column drain 90 is the first internal tank drain. Incomingfluid entering the tank 10 often contains some solids. These solids willaccumulate preferentially above the blanking plate 20. The center columndrain 90 is provided so the operator can drain this area. It should bedrained frequently until the water leaving the drain 90 runs clear.

In order to drain the solids that accumulate above the blanking plate20, it may also be desirable, in addition to the center column drain 90,to include a solids removal system 28, such as the Tore® solids removalsystem 28 to aid the center column drain 90 in removing solids from theinlet section 16 of the center column 14. A Tore® solids removal system28 is a solids hydro-transportation device that utilizes the naturalpower of a motive fluid, such as water, to mobilize and transportsolids, liquids or slurries. Tore® systems 28 are available from PDLSolutions Ltd. located in the United Kingdom. The Tore® solids removalsystem 28 includes a water inlet 94 that feeds water to the Tore® solidsremoval system 28 and a water and solids outlet 91 that drains a mixtureof water and solids out of the inlet section 16 of the center column 14.

Referring also to FIG. 6, the interface draw offs 92 collectivelyconstitute the second internal tank drain. As oil accumulates in thetank 10, it is common that some BS&W (basic sediment and water, aka“emulsion”) will accumulate immediately below or at the oil—waterinterface 48. The BS&W is heavier than pure oil because of the water andsolids contained in it. Therefore, the emulsion will build downward fromthe level of the normal oil-water interface 48. Several interface drawoffs 92 are provided in the tank 10 about a foot below the normaloil-water interface 48. Each interface draw offs 92 is constructed of anupper round horizontal draw off baffle 96 and a lower round horizontaldraw off baffle 98, with each draw off baffle 96 and 98 beingapproximately 24 inch in diameter. The upper draw off baffle 96 isstacked on top of the lower draw off baffle 98 of each interface drawoff 92 and the two draw off baffles 96 and 98 are spaced approximately 4inches apart. The area between the upper and lower draw off baffles 96and 98 is open to the interior 100 of the tank 10. A draw off pipe 102is connected to each of the lower draw off baffles 98, and theindividual draw off pipes 102 are connected together and piped to aconvenient elevation near the bottom of the tank 10 where the pipe exitsthe tank 10 as the BS&W interface drain 104. A BS&W valve (notillustrated) is installed to open and closed the BS&W interface drain104. When the BS&W valve on the BS&W interface drain 104 is opened, theBS&W layer flows horizontally between the upper and lower draw offbaffles 96 and 98 of each interface draw off 92 and out of the tank 10through the BS&W interface drain 104. When either clean water or cleanoil is observed in the sample of the outlet fluid, the BS&W has beenremoved and the BS&W valve can then be closed.

The upper and lower flow diverting baffles 54 and 24 and the interfacedraw offs 92 are supported within the tank 10 by support legs 106 thatextend down to the bottom 26 of the tank 10.

The tank 10 is provided with a cleanout man way 108 for providing accessto the interior 100 of the tank 10 when it is out of service and also aheater man way 110 for installation of an immersion heater (notillustrated) within the tank 10.

Although not specifically illustrated, sand removal systems can also beincluded in the bottom of the tank 10. These should be drained dailyuntil clean water is observed.

Although not illustrated, a water leg will be installed on site with thetank 10 as a means of regulating the fluid levels. The water leg is apipe within a pipe. The clarified water enters through the outer pipeand turns upward where it flows in the annular space between the twopipes. The inner pipe is sized for its circumference. The circumferenceof the outer pipe forms a spillover weir for the water with the innerpipe. The height of the top of the inner pipe establishes the weir thatsets the level of the oil-water interface 48 inside the separation tank10. The height of this weir is critical. It is always adjustable, eitherby removing an upper removable center pipe nipple, or via an externaladjustment assembly that slides a movable upper section of the innerpipe up and down to change its spillover elevation.

Installation and Operational Considerations

The separation tank 10 should have the gas phase piped to all othertanks being fed by the separation tank 10, including all oil tanks. Ifall tanks are not equalized, the gravity flow hydraulics will bedisrupted and the tank 10 could overflow. Also, it is important toequalize all separation tanks 10 in a tank battery to assure properflow.

The separation tank 10 is designed to operate when it is set straightand on true level. When this is not the case, the effluent qualitieswill suffer. Thus, it is important that all tanks 10 are set straightand level on a flat and level grade. It is recommended that the gradesbe shot with a transit before setting the tanks 10.

Also, although not illustrated, the correct spillover level of theinside pipe in the tank's water leg is calculated to within a fractionof an inch. If the water leg is set on a different elevation than itassociated separation tank 10, the levels will be incorrect, andperformance efficiencies will suffer dramatically.

When a new separation tank 10 is first put in service, it may take awhile for it to accumulate enough oil to begin to put oil into asubsequent sales oil tank. Once it does, the lease operator should colorcut the tank 10 to determine the thickness of the oil layer 50. It istypically 4-6 feet depending on the oil gravity, with heavier oilproducing a thicker oil layer 50.

The oil layer 50 should be maintained at the desired elevation. If itdeviates more than 12 inches from the desired elevation, the water legmust be adjusted to bring it into normal tolerances. In salt waterdisposal (SWD) plant applications, the trucked-in water may vary in itsweight or specific gravity. In these instances, the water leg may needto be adjusted daily to optimize oil recovery. If this is the case, andthe separation tank 10 is to be provided with a removable upper insidepipe water nipple, either several different length nipples should be cutand kept close by the water leg or the operator should order an externaladjustment assembly and retrofit the water leg to make this adjustmentfast and easy.

Sometimes the oil or water leaving the tank 10 may have excess BS&W,solids, or other contaminants due to the chemistry of the water.Physical separation systems, such as employed by the present invention,cannot compensate for this sort of issue. Thus, when this situation isobserved, the assistance of a local oilfield chemical company must besecured.

Cold temperatures may cause oil to congeal and get so thick that watercannot separate from it. Applying heat will resolve this, but hot oilingfiberglass vessels, such as the present tank 10, is discouraged unlessthe tank 10 is constructed of high temperature resin. Otherwise, themaximum recommended temperature for fiber reinforced plastic (FRP) tanksis 120 degrees F. The best remedy is to transfer the oil to a separatesteel tank and heat the oil in that separate tank before it enters thepresent separation tank 10.

Also, water, particularly fresh water, will freeze in many parts of theworld due to the cold weather conditions encountered in those areas.Immersion heaters installed in the tank 10 near the bottom 26 of thetank 10 will prevent this. A heater man way 110 is provided in the tankfor installing an immersion heater. Also, recirculating water constantlywithin the tank 10 will help prevent freezing. The greater thecirculation rate, the lower the freeze point will be.

Since the present separation tank 10 is designed to prevent overflowing,if an overflow event occurs, the probable cause is either a closed valveor a plugged line. The tank 10 is provided with an emergency oiloverflow outlet 112 for allowing oil to flow out of the tank 10 in theserare situations.

While the invention has been described with a certain degree ofparticularity, it is manifest that many changes may be made in thedetails of construction and the arrangement of components withoutdeparting from the spirit and scope of this disclosure. It is understoodthat the invention is not limited to the embodiments set forth hereinfor the purposes of exemplification, but is to be limited only by thescope of the attached claim or claims, including the full range ofequivalency to which each element thereof is entitled.

What is claimed is:
 1. A separation tank with enhance particulateremoval for separating gas, water and particulates from crude oilcomprising: a tank provided internally with a gas section at its top anda water section at its bottom with an oil section located between thegas section and the water section, means for discharging gas from thegas section, means for discharging particulates from the tank, said tankprovided with a vertical center column, said center column divided intoan inlet section and an outlet section, an inlet pipe attached to thecenter column offset so that fluid entering the inlet section from theinlet pipe swirls in a circular fashion within the center column, aspiral swirl vane diffuser provided in the center column above the inletpipe so that liquid flows out of the center column via the diffuser inan outwardly spinning and ever increasing radius spiral to slow thevelocity of the fluid and increase its effective separation time withinthe water section, a circular oil collector weir provided at a top ofthe oil section of the tank to remove oil from the oil section anddischarge it from the tank, an upper flow diverting baffle located belowthe spiral diffuser and a lower flow diverting baffle provided below theupper flow diverting baffle so that fluid flows downward within thewater section around the two flow diverting baffles, and outlet holesprovided in the outlet section of the center column under the lower flowdiverting baffle such that fluid flows through the outlet holes into theoutlet section to exit the tank.
 2. A separation tank according to claim1 wherein the means for discharging particulates from the tank furthercomprises: a center column drain provided at the bottom of the inletsection for removing solids that settle out of the crude oil in theinlet section.
 3. A separation tank according to claim 1 wherein themeans for discharging particulates from the tank further comprises: asolids removal system provided at the bottom of the inlet section forremoving solids that settle out of the crude oil in the inlet section.4. A separation tank according to claim 3 wherein the solids removalsystem further comprises: a solids hydro-transportation device thatutilizes the natural power of a motive fluid to mobilize and transportsolids, liquids or slurries.
 5. A separation tank according to claim 4wherein the solids removal system further comprises: a water inletconnected to the solids hydro-transportation device to feed water to itand a water and solids outlet connected to the solidshydro-transportation device that discharges a mixture of water andparticulates out of the tank.
 6. A separation tank according to claim 1further comprising: a blanking plate provided within the central columndividing said central column into the inlet section that extends fromthe top of the tank down to the blanking plate and the outlet sectionthat extends from the blanking plate down to the bottom of the tank. 7.A separation tank according to claim 1 wherein the means for discharginggas from the gas section further comprises: gas holes provided in thetop of the center column to allow gas to exit the center column andenter the gas layer of the tank, and a gas vent provided in the top ofthe tank and communicating with the gas layer for venting gas from thetank.
 8. A separation tank according to claim 1 further comprising: anoil conduit provided extending from the bottom side of the upper flowdiverting baffle up into the oil layer below the circular oil collectorweir.
 9. A separation tank according to claim 1 further comprising:interface draw offs provided within the tank below an oil-waterinterface, and said interface draw offs communicating with a interfacedrain for removing excess BS&W from the tank.
 10. A separation tankaccording to claim 9 wherein each interface draw off further comprises:an upper draw off baffle separated from a lower draw off baffle with thearea between the two draw off baffles open to the interior of the tank,a draw off pipe is connected to each of the lower draw off baffles, thedraw off pipe connected to a common draw off pipe that exits the tank asthe BS&W outlet, and a BS&W valve attached to the BS&W outlet to openand close the BS&W outlet.
 11. A separation tank according to claim 1wherein the spiral van diffuser further comprises: spaced apart verticalcurved inlet diverters secured between a horizontal quieting lower donutbaffle and a horizontal quieting upper donut baffle, and inlet fluidslots provided between adjacent inlet diverters such that the inletfluid slots communicate with the inlet section of the center column toallow fluid to flow out of the center column between the inlet divertersand into the water section of the tank.
 12. A separation tank accordingto claim 1 further comprising: the upper flow diverting baffle is convexon its top side and is concave on its bottom side.
 13. A separation tankaccording to claim 2 further comprising: the lower flow diverting baffleis convex on its top side and is concave on its bottom side.
 14. Aseparation tank according to claim 3 further comprising: oil-dedicatedweep holes provided extending through the top of the lower flowdiverting baffle.